Heretofore it has been the practice in clean rooms to direct a constant stream of laminar air under pressure through a zone containing the product or other object to be protected from contamination and take steps to insure against turbulence. This laminar air stream usually is achieved by directing air at constant velocity via High Efficiency Particulate Air (HEPA) filters and diffusers mounted in the ceiling downwardly past the object or area to be protected, through apertures in a preferably grated floor then via return ducts back to the ceiling and through the HEPA filters for substantially continual recirculation.
U.S. Pat. Nos. 3,367,257, 4,100,347 and 4,137,831 disclose clean rooms or other substantially closed environments in which filtered air is diffused and directed as a laminar air stream through a work zone. Also, Ashrae Journal, August 1962, p. 37, "Jet Stream Ventilation for Extreme Air Cleanliness" discloses a hospital operating room in which a sterile zone of completely filtered air is provided around the patient by a recirculating flow of air that "should be quite laminate and hence have relatively thick boundary layers".
Clean rooms of this Vertical Laminar Flow (VLF) type operate very satisfactorily and provide air in the work zone that is as clean as can be supplied by the HEPA filters used. It is therefore preferred for those clean rooms classified under U.S. Federal Standard 209B as Class 10, Class 100 or even Class 1,000. However, these VLF systems are very expensive because of the large number of HEPA filters needed.
To reduce cost, there is a need for a novel approach to clean room contamination control that is especially suited for clean rooms classified as Class 10,000 or Class 100,000; i.e., those that do not have to be maintained "super clean".